Fifteen years ago, Pluto was kicked out of the planet club. On August 24, 2006, members of the International Astronomical Union voted in favor of a new definition of what constitutes a planet. What was once considered the solar system’s ninth planet no longer qualified.
The public outcry was immediate. “In changing the definition of planet, the International Astronomical Union is messing with something much bigger than it is,” one Science News reader complained in a letter to the editor. “Think of all the dictionaries, encyclopedias, textbooks and websites that will need revision.”
Many planetary scientists were upset too, though mostly for other reasons. As astronomy writer Lisa Grossman explains in this issue, some researchers thought the new definition — that a planet is a spherical body that orbits the sun (which Pluto is) and has cleared other objects out of its orbit (which Pluto hasn’t) — was too restrictive. Some scientists still lobby for a broader definition.
The debate over classifying solar system objects reminds me of another classification challenge: categorizing the diversity of life on Earth. As a child, I remember learning about the five kingdoms of life: animals, plants, fungi, protists (things like algae) and monerans (bacteria). Though there was never a vote (that I’m aware of), that system was superseded. In the 1990s, biologists proposed new groups such as domains — bacteria, archaea (unicellular organisms that had once been considered bacteria) and eukaryotes. More recently, scientists have debated how to organize eukaryotes, organisms that store DNA within a cell nucleus. For example, did you know you’re an opisthokont? According to some biologists, all animals, fungi and some single-celled eukaryotes fall into that “supergroup” (SN: 8/8/15, p. 22). The name, roughly meaning “rear pole,” references the fact that many opisthokonts have at least some cells powered by a whiplike tail. Animals, for instance, have sperm. But I doubt many schoolchildren would cry if we lost our membership in this group, as they did over Pluto’s “demotion.”
Categorizing life is an ongoing process, and even the definition of life itself is uncertain, with questions about whether viruses should be considered alive. Still, classification is more than just semantics. These groupings serve a purpose; they reveal our place in nature and help us understand how life has evolved. As new information, particularly genetic information, has come in, scientists have realized that some groupings had been based on superficial similarities and have revised the taxonomy accordingly.
Classification holds a similar purpose in planetary science and is also subject to new information. As Grossman shows, the 2006 vote didn’t end debate over what defines a planet. Some scientists now argue that interesting geology is the true hallmark, which would make many solar system objects eligible for a status upgrade. Though this broader definition of a planet may never be put to a vote, we’d love to know whether you think it’s a plausible alternative. Let us know at email@example.com.